1. Field of the Invention
The present invention relates to a photoelectric conversion material containing a polymer having a condensed aromatic ring structural unit, a method for producing the photoelectric conversion material, and an organic photovoltaic cell using the photoelectric conversion material.
2. Description of the Related Art
Organic photovoltaic cells using organic materials, which can be easily produced by a low-cost process such as a roll-to-roll process, have attracted much attention. Such organic photovoltaic cells include bulk heterojunction-type organic photovoltaic cells (hereinafter referred to also as BHJ solar cells).
The BHJ solar cell has a photoelectric conversion layer for converting a light into electricity, and the photoelectric conversion layer contains a mixture of a donor domain and an acceptor domain. The donor domain contains a photoelectric conversion material that acts as an electron donor, and the acceptor domain contains a photoelectric conversion material that acts as an electron acceptor. Specifically, the photoelectric conversion layer is interposed between positive and negative electrodes. When sunlight is introduced through the positive electrode into the photoelectric conversion layer, an exciton is generated in the layer.
The exciton reaches an interface between the donor and acceptor domains, and then is divided into an electron and a hole. The electron is transferred through the acceptor domain to the negative electrode, while the hole is transferred through the donor domain to the positive electrode. The hole and the electron are utilized to generate an electrical energy for energizing an external circuit electrically connected to the negative and positive electrodes.
As described in Japanese Laid-Open Patent Publication No. 2007-273939, typical examples of the photoelectric conversion materials, i.e., the donor and the acceptor, used in the photoelectric conversion layer having the above function include poly(3-hexylthiophene) (P3HT, see FIG. 7) and phenyl-C61-butyric acid methyl ester (PCBM, see FIG. 8) respectively.
P3HT and PCBM have energy levels of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) shown in FIG. 9. When the light is introduced into the photoelectric conversion layer as described above, an electron is transited from the HOMO to the LUMO in the P3HT functioning as a donor. Thus, the energy level difference between the HOMO and the LUMO of P3HT corresponds to the bandgap (Eg).
Then, the electron transited to the LUMO of P3HT is transferred to the LUMO of the PCBM functioning as an acceptor, whereby the electron and the hole are generated. Thus, the energy level difference between the LUMO of P3HT and the LUMO of PCBM results in the energy loss, and the energy level difference between the HOMO of P3HT and the LUMO of PCBM corresponds to the open circuit voltage (Voc).
As the photoelectric power conversion efficiency of the solar cell becomes higher, the area required for achieving a desired power generation capacity is reduced. Therefore, advantageously, the weight and the installation area of the solar cell can be reduced, so that the layout of the solar cell can be designed more freely.
The photoelectric power conversion efficiency of the organic photovoltaic cell such as the BHJ solar cell can be increased by (a) improving the absorption of a light to accelerate the exciton generation, (b) improving the absorption of a long-wavelength (near-infrared) light to increase the sunlight utilization efficiency, (c) increasing the open circuit voltage Voc, etc. In view of achieving the procedures of (a) to (c), a donor having (A) a high absorbance coefficient, (B) a small energy level difference between the HOMO and LUMO (a narrow bandgap Eg), and (C) a LUMO energy level close to that of the acceptor may be selected.
Condensed aromatic ring compounds (i.e., π-electron conjugated compounds) described in Japanese Patent No. 4005571 and Japanese Laid-Open Patent Publication Nos. 2010-056492, 2007-019086, and 2010-508677 (PCT) may have the above properties of (A) to (C). Such condensed aromatic ring compounds are sometimes referred to as graphenes (see Japanese Laid-Open Patent Publication No. 2007-019086).